2.1.2: Discovery of Subatomic Particles and the Bohr Atom

• When a high-voltage electrical discharge is passed through a sample of hydrogen gas at low pressure, the result is individual isolated hydrogen atoms that emit a red light.
• The color of the light emitted by the hydrogen atoms does not depend greatly on the temperature of the gas in the tube.
• When the emitted light is passed through a prism, only a few narrow lines of particular wavelengths, called a line spectrum, are observed rather than a continuous range of wavelengths.
• The light emitted by hydrogen is red because, of its four characteristic lines, the most intense line in its spectrum is in the red portion of the visible spectrum.
• Johann Balmer showed that the frequencies of the lines observed in the visible region of the hydrogen line spectrum fit a simple equation where n = 3, 4, 5, 6. These lines are known as the Balmer series.
• Johannes Rydberg restated and expanded Balmer's result in the Rydberg equation where n1 and n2 are positive integers, nh>nl, and Rh (the Rydberg constant) has a value of 1.09737 x 10^7 m-1.
• Rydberg's equation described the wavelengths of the visible lines in the emission spectrum of hydrogen and predicted the wavelengths of other series of lines that would be observed.
• Niels Bohr proposed a theoretical model for the hydrogen atom that explained its emission spectrum.
• Bohr's model assumption: the electron moves around the nucleus in circular orbits that can have only a certain allowed radii and that the electron could occupy only certain regions of space.
• As n decreases, the energy holding the electron and the nucleus together becomes increasingly negative; the radius of the orbit shrinks and more energy is needed to ionize the atom.
• The orbit with n = 1 is the lowest lying and most tightly bound.
• Because a hydrogen atom with its one electron in this orbit has the lowest possible energy, this is the ground state.
• The ground state is the most stable arrangement of electrons for an element or compound.
• A hydrogen atom with an electron in an orbit with n > 1 is in an excited state, any arrangement of electrons that is higher in energy that the ground state.
• When an atom in an excited state undergoes a transition to the ground state (decay), it loses energy by emitting a photon whose energy corresponds to the difference in energy between the two states.
• Electrons can occupy only certain regions of space, called orbits.
• Orbits closer to the nucleus are lower in energy.
• Electrons can move from one orbit to another by absorbing or emitting energy, giving rise to characteristic spectra.
• If white light is passed through a sample of hydrogen, hydrogen atoms absorb energy as an electron is excited to higher energy levels.
• If the light that emerges is passed through a prism, it forms a continuous spectrum with black lines corresponding to transitions.
• Any given element has a characteristic emission spectrum and a characteristic absorption spectrum, which are complementary images.
• Emission and absorption spectra form the basis of spectroscopy, which uses spectra to provide information about the structure and composition of a substance or object.
• An electrical discharge excites neutral atoms to a higher energy state and light is emitted when the atoms decay to the ground state.